Developing Ni single-atom sites in carbon nitride for efficient photocatalytic H2O2 production

Photocatalytic two-electron oxygen reduction to produce high-value hydrogen peroxide (H2O2) is gaining popularity as a promising avenue of research. However, structural evolution mechanisms of catalytically active sites in the entire photosynthetic H2O2 system remains unclear and seriously hinders the development of highly-active and stable H2O2 photocatalysts. Herein, we report a high-loading Ni single-atom photocatalyst for efficient H2O2 synthesis in pure water, achieving an apparent quantum yield of 10.9% at 420 nm and a solar-to-chemical conversion efficiency of 0.82%. Importantly, using in situ synchrotron X-ray absorption spectroscopy and Raman spectroscopy we directly observe that initial Ni-N3 sites dynamically transform into high-valent O1-Ni-N2 sites after O2 adsorption and further evolve to form a key *OOH intermediate before finally forming HOO-Ni-N2. Theoretical calculations and experiments further reveal that the evolution of the active sites structure reduces the formation energy barrier of *OOH and suppresses the O=O bond dissociation, leading to improved H2O2 production activity and selectivity.

application of high-purity H 2 O 2 (avoiding complicated and expensive distillation purification), is one of the goals pursued in this field 4,9,10,[13][14][15][16][17] .Among various photocatalysts, the low-cost graphitic carbon nitride (g-C 3 N 4 ) has shown great potential for H 2 O 2 production due to the certain light-responsive ability, suitable energy band structure, and metal-free structure suppression of H 2 O 2 surface decomposition 10,14,[18][19][20][21] .However, the highest solar-to-chemical conversion (SCC) efficiency of the currently developed g-C 3 N 4 -based photocatalysts for H 2 O 2 production in pure water is still less than 0.65% 10,13,14,[22][23][24] , and the unsatisfactory catalytic activity restricts the industrial production and practical application of photocatalytic H 2 O 2 synthesis.Therefore, further development of highly active photocatalysts for H 2 O 2 production in pure water is of great significance and presents challenges.
The photocatalytic activity is considered to be the cumulative result of surface reactions between the active sites on the catalyst and the reactants 25,26 .Several improved strategies (such as introducing surface defects 14,27 , doping atoms 18,28,29 , designing donor-acceptor units 13,15,30 , forming heterojunctions 11,22,31 , and developing metal/ organic frameworks [32][33][34][35] , etc.) have been developed to enhance photocatalytic H 2 O 2 activity.However, due to the structural complexity caused by these strategies, identifying the dynamic structural evolution of active sites in photocatalytic surface reaction and elucidating the corresponding catalytic enhancement mechanism remains a great challenging and, as a result, is rarely reported.In pure water system, understanding how the active sites specifically participate in O 2 adsorption and activation during photoactivation at the atomic scale is a fundamental prerequisite for further enhancing the activity of 2e - ORR, which is crucial for the rational development of highperformance 2e -ORR photocatalysts.Excitingly, single-atom photocatalysts (SAPs) with well-defined single-atom active sites and high atomic utilization, serving as idealized catalytic models, provide opportunities for in-depth exploration of the active sites structure evolution and the reaction mechanism [36][37][38] .Although SAPs have achieved some promising results in 2e -ORR, it still faces the following key problems: (1) Most studies ignore the microscopic structural control of semiconductor substrates in SAPs, resulting in low singleatom loading that cannot provide abundant active sites, making the H 2 O 2 generation activity in pure water unsatisfactory.(2) More urgently, the dynamic structural evolution of the active sites and corresponding catalytic enhancement mechanism in photocatalytic 2e -ORR under practical reaction conditions remain unclear, severely limiting the further design and development of highly active photocatalysts for H 2 O 2 synthesis in pure water.
Herein, we report a general method (by tuning the substrate microstructure and optimizing the loading process) for the synthesis of SAPs with high single-atom loading based on g-C 3 N 4 , and successfully synthesize a series of high-loading M-SAPs (M=Fe, Co, Ni, Cu, Zn, Sr, W, Pt) with porous ultrathin structures.Benefiting from the highconcentration single-atom sites exposed by this structure, the developed high-loading Ni single-atom photocatalyst (Ni SAPs -PuCN) exhibits high activity and selectivity for H 2 O 2 generation.Notably, in pure water, the apparent quantum yield (AQY) at 420 nm reaches 10.9% while achieving a high SCC efficiency of 0.82%, which is the most efficient g-C 3 N 4 -based photocatalyst for H 2 O 2 production reported so far.Pioneeringly, combining in situ synchrotron X-ray absorption spectroscopy, Raman spectroscopy, and theoretical calculation, we directly observed the transformation of initial Ni-N 3 , which is the core factor for the high activity and selectivity H 2 O 2 production of Ni SAPs -PuCN.Revealing the catalytic enhancement mechanism through the dynamic structural evolution of active sites provides insights for the development of highly active photocatalysts and a deeper understanding of photocatalysis.

Results and discussion
A general synthesis strategy for high-loading M-SAPs Increasing the loading of single atoms to create more active sites is beneficial to improve the catalytic activity 39,40 .A schematic diagram illustrating the synthesis of high-loading M-SAPs is presented in Fig. 1a, along with the presumed structural changes in the heptazine unit of the corresponding g-C 3 N 4 .Briefly, the synthesis of high-loading M-SAPs is primarily divided into two steps: regulating the microtopography and further optimizing the loading process (involving continuous ultrasonic treatment in wet-chemical precipitation).Firstly, the substrate microtopography was adjusted by thermal stripping and ultrasonic exfoliation of the original bulk g-C 3 N 4 (denoted as BCN), thereby preparing porous ultrathin g-C 3 N 4 nanosheets (denoted as PuCN), which are beneficial for providing more sites for single-atom loading.Next, PuCN was further loaded with metal single atoms by wetchemical precipitation under continuous ultrasonic conditions.The continuous ultrasonic treatment can not only promote the uniform dispersion of single atoms, but also further exfoliate g-C 3 N 4 (destroy the van der Waals forces between carbon nitride layers) to provide abundant loading sites, and finally achieve a high-loading M-SAPs with a porous ultrathin structure (denoted as M SAPs -PuCN, see "Methods" for details).Importantly, we further confirmed that this synthetic strategy is applicable for the preparation of a series of high-loading M SAPs -PuCN (M=Fe, Co, Ni, Cu, Zn, Sr, W, Pt).The aberration-corrected high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) images in Fig. 1b-i reveal the uniform dispersion of high-density metal single atoms (Fe, Co, Ni, Cu, Zn, Sr, W, Pt) on g-C 3 N 4 , with no nanoclusters or particles observed.This observation is consistent with the X-ray diffraction (XRD, Supplementary Figs. 1, 2) results of all samples.Energy-dispersive X-ray spectroscopy (EDS, Supplementary Figs.3-9) mapping further shows that C, N and M elements were uniformly distributed on g-C 3 N 4 .The content of metal loading in M SAPs -PuCN was all above 10 wt% by inductively coupled plasma mass spectrometry (ICP-MS, Supplementary Table 1).

Structural characterization of Ni SAPs -PuCN
It was found that Ni SAPs -PuCN has the highest photocatalytic H 2 O 2 generation activity among the prepared various M SAPs -PuCN (Supplementary Fig. 10); therefore, the structure of Ni SAPs -PuCN was further characterized in detail.The microstructure of the samples was investigated using HAADF-STEM, atomic force microscopy (AFM) and scanning electron microscopy (SEM).In Fig. 2a, BCN exhibited a thicklayered bulk structure, while the PuCN showed a distinct thin layer and porous structure (Fig. 2b).After the ultrasonic-wet chemical loading of Ni single atoms, more abundant pores and thinner undulating folds in Ni SAPs -PuCN can be clearly seen (Fig. 2c-d), which are also verified in AFM images (Supplementary Fig. 11).The decrease of the (002) peak intensity of Ni SAPs -PuCN in XRD (Supplementary Fig. 2) indicates the weakening of the interlayer stacking, which corresponds to the ultrathin structure.Meanwhile, SEM and TEM (Supplementary Fig. 12), N 2 physisorption measurements and pore size distribution (Supplementary Fig. 13 and Supplementary Table 2) together showed that the porous ultrathin structure of Ni SAPs -PuCN had a significantly increased specific surface area (139.6 m 2 g −1 , about 8.3 times of BCN) and abundant pore distribution.This adjusted structure would be very favorable for anchoring single atoms, as confirmed in the subsequent HAADF-STEM characterization (Fig. 2e and g).In Fig. 2e, numerous isolated bright spots clearly differ from the g-C 3 N 4 substrate contrast (See Supplementary Fig. 14a-c for more details), indicating that the high density of Ni single atoms was successfully dispersed on the Ni SAPs -PuCN without any nanoparticles or clusters being found.The EDS mapping (Fig. 2f) showed that Ni, C and N elements were uniformly distributed on the ultrathin porous substrate.As shown in Fig. 2g, highdensity Ni single atoms are anchored around the pores of Ni SAPs -PuCN, which is favorable for the adsorption and activation of reaction gases in the shuttle pores.The Ni content in Ni SAPs -PuCN was as high as 12.5 wt% measured by ICP-MS.In addition, the Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS) demonstrated that this synthesis method maintains the structure of g-C 3 N 4 (Supplementary Fig. 15).Therefore, based on the above systematic characterizations, high-density Ni SAPs -PuCN with porous ultrathin structure was successfully synthesized, and the schematic structure is shown in Fig. 2h.
Furthermore, the coordination structure of Ni in Ni SAPs -PuCN was confirmed by X-ray absorption fine structure spectroscopy (XAFS).Figure 3a shows the Ni K-edge X-ray absorption near-edge structure (XANES) spectra of Ni SAPs -PuCN and Ni foil, NiO, and NiPc as comparisons.The absorption edge position of Ni SAPs -PuCN was located between Ni foil and NiO, suggesting that the valence state of Ni in Ni SAPs -PuCN was between 0 and +2.The Fourier transform of the extended X-ray absorption fine structure (FT-EXAFS) spectra of the samples is shown in Fig. 3b.The Ni SAPs -PuCN exhibits a main peak around 1.69 Å (Fig. 3b), which is mainly attributed to the scattering interaction between Ni atoms and the first layer (Ni-N).However, no peak of Ni-Ni bond at about 2.17 Å was observed in Ni SAPs -PuCN compared to Ni foil.This indicates that Ni species exists in the form of single atoms in Ni SAPs -PuCN, validating the results of HAADF-STEM and XRD.Moreover, the wavelet transform (WT) in the Ni K-edge EXAFS further analyzes the coordination environment of Ni in Ni SAPs -PuCN.Unlike Ni foil and NiO (Fig. 3f and Fig. 3g), the WT contour plot of Ni SAPs -PuCN (Fig. 3e) shows that there is only one intensity maximum around 5.23 Å −1 (attributed to Ni-N coordination), indicating Ni sites are atomically dispersed on Ni SAPs -PuCN.The best-fit analysis results of EXAFS (Fig. 3d and Supplementary Table 3) show that each Ni atom in Ni SAPs -PuCN is bonded with three N atoms as Ni-N 3 coordination, and the average Ni-N bond length is about 2.07 Å.The inset of Fig. 3c shows the Ni-N 3 coordination structure model in Ni SAPs -PuCN, and the simulated XANES spectra base on this agree well with the experimental results (Fig. 3d), illustrating the rationality of the Ni-N 3 sites.Moreover, further theoretical calculations (Supplementary Fig. 16) also support the fitting results of Ni-N 3 .In addition, thermodynamic and kinetic calculation results (Supplementary Fig. 14d-g experimental characterization results, in which Ni atoms exist as isolated single atoms rather than in the form of clusters.

Catalytic activity and selectivity of Ni SAPs -PuCN for photocatalytic H 2 O 2 production
The photocatalytic H 2 O 2 generation performance of the samples was evaluated in O 2 -saturated pure water (without any sacrificial agent or buffer salt solution; pH = 7) under visible light irradiation (λ ≥ 420 nm).
As shown in Fig. 4a, the H 2 O 2 generation rate of PuCN (41.1 μmol L -1 h -1 ) was slightly increased relative to BCN (16.5 μmol L -1 h -1 ), which could be the porous ultrathin structure expanded the contact range of O 2 .
Notably, the Ni SAPs -PuCN exhibited significantly enhanced H 2 O 2 generation rate up to 342.2 μmol L -1 h -1 (Fig. 4a), which was 20.7 and 8.3 times of BCN and PuCN, respectively (Fig. 4b).This indicates that the Ni single atoms are the core of the enhanced H 2 O 2 production activity.Meanwhile, the effect of Ni single atoms loading content and Ni nanoparticles on the H 2 O 2 activity was further sorted out (Supplementary Figs.17, 18).Furthermore, the H 2 O 2 generation activity of Ni SAPs -PuCN can be improved to 640.1 μmol L -1 h -1 under AM 1.5 G irradiation (Fig. 4b).The variable comparison and radical trapping experiments.
(Supplementary Fig. 19) confirm that H 2 O 2 is indeed generated by the 2e -ORR pathway.The oxidation reaction of photogenerated holes was confirmed by photocatalytic O 2 production test (Supplementary Fig. 20), which is consistent with previous reports 10 .The final yield of photocatalytic H 2 O 2 production depends on the formation and decomposition rates of H 2 O 2 , and the experiment (Supplementary Fig. 21) suggested that the decomposition rate of H 2 O 2 on the samples is relatively slow.A systematic performance comparison of Ni SAPs -PuCN with recently reported g-C 3 N 4 -based materials and other types of materials for photocatalytic H 2 O 2 production in pure water is presented in Supplementary Table 4.The comparison of corresponding normalized H 2 O 2 yields (μmol g −1 h −1 ) in Supplementary Fig. 22 demonstrates the efficient H 2 O 2 generation activity of Ni SAPs -PuCN.
To further evaluate the light utilization efficiency of Ni SAPs -PuCN in pure water, the AQY was measured under monochromatic light irradiation (Fig. 4c and Supplementary Table 5).The AQY of Ni SAPs -PuCN at 420 nm reaches 10.9%, surpassing most of reported g-C 3 N 4based photocatalysts and currently developed materials (Supplementary Fig. 23 for AQY comparison and Supplementary Table  highest value reported so far for g-C 3 N 4 -based photocatalysts in pure water (Fig. 4g for SCC efficiency comparison and Supplementary Table 4).Simultaneously, in Fig. 4g, when compared with other types of photocatalysts currently developed, the SCC efficiency of Ni SAPs -PuCN still demonstrates advantages and approaches the highest SCC efficiency values (1.0%-1.2%)reported for powder photocatalyst 15,30 .
Overall, according to the standard H 2 O 2 yield, AQY and SCC efficiency as the evaluation indicators of photocatalytic activity, Ni SAPs -PuCN has excellent H 2 O 2 generation activity in pure water.Meanwhile, Ni SAPs -PuCN exhibited excellent performance cyclability (Supplementary Fig. 24) and structural stability (Supplementary Fig. 25a-f).The thermodynamic stability of the Ni SAPs -PuCN model was also confirmed by molecular dynamics (MD) calculations (Supplementary Fig. 25g).
The high ORR selectivity is the guarantee of high activity 2,4

Electronic Structure and carrier separation properties of Ni SAPs -PuCN
The electronic structure of photocatalysts largely determines the carrier separation characteristics and further affects the surface reaction efficiency 19,26,41,42 .In the UV-Vis diffuse reflectance spectroscopy (DRS) of the samples (Supplementary Fig. 26a), the introduction of Ni single atoms expands the absorption of visible light and adjusts the band structure.The introduction of Ni single atoms effectively promotes carrier separation and transport, as confirmed by a series of spectroscopic measurements (Supplementary Fig. 27).Next, femtosecond transient absorption spectroscopy (fs-TAS) was used to further investigate the kinetic behaviors of the photogenerated carriers.The results indicate that Ni SAPs -PuCN has a shorter lifetime than BCN (See Supplementary Fig. 28 for more details), which could be attributed to the deep trapping sites induced by Ni single atoms and has been demonstrated to facilitate the 2e -ORR process 10 .Bader charge analysis (Supplementary Fig. 29) indicates that the chemical valence of Ni single-atom in Ni SAPs -PuCN is positive, which is in line with the results of Ni 2p XPS (Supplementary Fig. 15d) and XANES (Fig. 3a).The total density of states (TDOS) of BCN exhibits a typical semiconducting nature (Supplementary Fig. 30a), where the valence band (VB) are mainly composed of N 2p orbitals, while the conduction band (CB) is mainly composed of C 2p and N 2p orbitals 43,44 .As for Ni-SAPs -PuCN, the introduction of Ni atoms creates impurity levels and narrows the band gap (Supplementary Fig. 30b), which agrees well with the experimental results (Supplementary Fig. 26b).Combined with the projected density of states (PDOS) of Ni (Supplementary Fig. 31), the Ni 3d orbitals contribute to both CB and VB, implying that Ni single atoms greatly optimize the electronic structure.Further, in the charge density difference of Ni SAPS -PuCN (Supplementary Fig. 32), there are remarkably  34).Furthermore, this charge transport mechanism was confirmed again based on Bader charge analysis, where the adsorbed O 2 molecules gain electrons (0.48|e|) from Ni SAPs -PuCN (Fig. 5d).Based on the above results, to further  Next, as shown in Fig. 5a, when light sources are introduced to initiate the photocatalytic 2e -ORR, the Ni K-edge shifts to the lower energy direction (between the O 2 -saturated aqueous solution and the Ar-saturated aqueous solution), corresponding to a decrease in the oxidation state.This suggests that the 2e -ORR reaction is further driven (Ni(*) . Nonetheless, it is difficult to accurately capture the key intermediate *OOH on the Ni site using in situ XAFS, so the Raman spectroscopy (Fig. 5h) was used to further identify the reaction details.In Fig. 5h, Ni SAPs -PuCN gradually exhibits a new absorption band at 855 cm −1 in O 2 aqueous solution with illumination time, which can be attributed to the O = O stretching mode in Ni-OOH 10,45,46 .Moreover, theoretical calculation (Supplementary Fig. 35) further reveal that the *OOH on the Ni sites is in an end-on adsorption configuration (HOO-Ni-N 2 , Fig. 5c and Fig. 6h).Simultaneously, the •OOH radicals were also detected through EPR test 30,47 .In Fig. 5g, compared with BCN and PuCN, Ni SAPs -PuCN had a stronger •OOH signal under the same conditions, implying that Ni single atoms can effectively promote the •OOH generation.Throughout the in situ XAFS, the FT-EXAFS spectra (Fig. 5b) show that the main peak shifts (~0.03 Å) to longer lengths, implying the expansion of the Ni-N bonds during the reaction.Thus, combined with the above comprehensive analysis, we elucidated the structural evolution mechanism of Ni active sites on Ni SAPs -PuCN in photocatalytic 2e -ORR, and the corresponding schematic diagram is shown in Fig. 5c.In photocatalytic 2e -ORR, after O 2 adsorption and activation, the Ni-N 3 sites are transformed into O 1 -Ni-N 2 , which further promotes the formation of the key *OOH intermediate with end-on adsorption configuration (HOO-Ni-N 2 ), thereby accelerating the generation of H 2 O 2 .

In-depth exploration of Ni site structure evolution and catalytic enhancement mechanism
To further understand the relationship between structural evolution of Ni active sites and high H 2 O 2 activity, we systematically investigated the surface reaction mechanism of 2e -ORR by combining theoretical calculations and experiments.The first step of the 2e - ORR surface reaction is O 2 adsorption and activation.The O 2 adsorption configuration and the O 2 adsorption energy on the catalyst are particularly critical for the subsequent reactions 48,49 .The adsorption configuration of O 2 on the catalyst is categorized into three basic types (Fig. 6a, upper): Yeager type (side-on), Griffith type (side-on), and Pauling type (end-on) 48,50 .The end-on O 2 adsorption configuration, which tends to maintain the O = O bond, can inhibit the 4e -ORR (O 2 + 4e -+ 4H + → 2H 2 O) and promote the highly selective 2e -ORR 7,48,49,51 .Based on this, we first performed first-principles calculations to investigate the O 2 adsorption and activation properties in this system.As shown in the calculation results in Fig. 6a (bottom), in comparison to BCN, O 2 was adsorbed by Ni single atoms on Ni SAPs -PuCN in an end-on configuration (O 1 -Ni-N 2 ), which is consistent with the fitted data from in situ XAFS (Fig. 5e).Simultaneously, the O 2 adsorption Gibbs free energy on Ni SAPs -PuCN (-0.83 eV, Fig. 6b) was much lower than that of BCN (1.10 eV, Fig. 6b Based on the above results, the free energy changes of the photocatalytic 2e -ORR on the BCN and Ni SAPs -PuCN were further calculated.In Fig. 6f, the introduction of Ni active sites greatly facilitates the O 2 adsorption compared to BCN (reduced from 1.10 eV to -0.83 eV, and the theoretical calculation of BCN for O 2 adsorption is in Supplementary Fig. 36).The formation and hydrogenation of the core intermediate *OOH are also the key to 2e -ORR 10,14  In summary, we present a general synthesis strategy for highloading M SAPs -PuCN (M = Fe, Co, Ni, Cu, Zn, Sr, W, Pt) with porous ultrathin structure.This approach can be applied to various catalytic reactions and energy conversion fields.The well-designed high-loading Ni SAPs -PuCN exhibits excellent photocatalytic H 2 O 2 performance, with Ni single atoms optimizing the electronic structure and providing numerous highly active reaction sites.Importantly, through in situ XAFS and theoretical calculations, we reveal the structural evolution of Ni single-atom active sites (Ni-N 3 → O 1 -Ni-N 2 → HOO-Ni-N 2 ) in surface reactions, closely related to the high catalytic activity.The O 1 -Ni-N 2 intermediate state structure ensures the proper O 2 adsorption configuration and energy, not only suppressing O 2 dissociation and enhancing the selectivity, but also promoting intermediate conversion to H 2 O 2 .This dynamic self-tuning of the coordination structural evolution significantly lowers the formation energy barrier of *OOH, which is a pivotal factor contributing to the high activity and selectivity of Ni SAPS -PuCN.Elucidating the mechanism of structural evolution closely associated with high catalytic activity paves the way for the design of highly efficient photocatalysts and a deeper comprehension of photocatalysis.

Methods
Synthesis of BCN 10 g of urea was placed in a covered alumina crucible and heated to 550 °C at a rate of 5 °C min −1 in a muffle furnace for 2 h.After the crucible was cooled to room temperature, the product was collected and ground into powder, washed several times with deionized water, and dried under vacuum at 80 °C for 12 h (the obtained BCN was about 300 mg).

Synthesis of PuCN
The PuCN was obtained by thermal stripping and ultrasonic stripping of muffle furnace was extended to 4 h.After cooling, 50 mg of the powder was ultrasonically stripped in 100 ml of pure water for 8 h and collected by centrifugation.The product was drop-coated evenly in glassware and dried overnight in vacuum (the obtained PuCN was about 45 mg).

Synthesis of Ni SAPs -PuCN and various M SAPs -PuCN
Put 50 mg of PuCN in 40 ml of pure water and add 10 ml of ethanol, first sonicate for 2 h to evenly disperse the sample.Then, under continuous ultrasonic and stirring, a certain amount of NiCl 2 •6H 2 O solution (45 mg of NiCl 2 •6H 2 O in 30 ml of solvent, 1.5 mg ml −1 ; water to ethanol in the solvent is 1:1) was slowly added dropwise, and ultrasonically treated for 3 h to ensure full contact between metal ions and PuCN (continuous sonication is beneficial to promote uniform dispersion and high loading of single atoms, see Supplementary Fig. 37 for more details).The volume of Ni solution added (5 ml, 10 ml or 15 ml, etc.) is controlled to adjust the metal loading.Then the mixed solution was heated in an oil bath at 60 °C for 4 h with vigorous stirring.The product was collected by centrifugation and dried overnight at 80 °C in a vacuum oven.The powder was fully ground and heated to 350 °C in an Ar atmosphere at 2 °C min −1 in a tube furnace and kept for 2 h, and finally cooled to room temperature.The collected powders were washed multiple times with 2% (v/v) HCl to remove nanoparticles or clusters, then washed at least 3 times with pure water and dried overnight in a vacuum oven at 80 °C to obtain Ni SAPs -PuCN (about 40 mg).The maximum Ni single-atom loading in Ni SAPs -PuCN is 12.5 wt %, and more details are shown in Supplementary Fig. 17 and Supple Photocatalytic H 2 O 2 production reaction 30 mg of photocatalyst was placed in 30 ml of pure water (pH = 7), and ultrasonically treated for 30 min until the catalyst powder was completely dispersed.The solution was continuously filled with O 2 for 1 h in the dark to saturate the O 2 .The reaction solution was irradiated with a 420 nm cut-off film (λ ≥ 420 nm) under a 300 W Xe lamp (PLS-SXE 300D/DUV, PerfectLight) and started the photoreaction test under magnetic stirring.The flow rate of the O 2 was 50 ml min -1 and the reaction temperature was controlled at 25 °C by circulating water.The average light intensity was 60 mW cm -2 .Every 15 min, 2 ml of the reaction solution was taken out, and the photocatalyst was removed by centrifugal filtration.The H 2 O 2 content generated by the reaction was detected by iodometric method 14,28 .Briefly, 0.5 ml of 0.4 mol L -1 potassium iodide (KI) solution and 0.5 ml of 0.1 mol L -1 potassium hydrogen phthalate (C 8 H 5 KO 4 ) solution was added to 1 ml obtained solution and kept for 30 min.The content of H 2 O 2 was determined by absorbance at 350 nm with a UV-Vis spectrophotometer (The standard curve for H 2 O 2 was shown in Supplementary Fig. 38).The effect of different catalyst concentrations on the photocatalytic H 2 O 2 activity was also illustrated (Supplementary Fig. 39).More details of the process can be found in our previous paper 14 .The photocatalytic O 2 generation reaction was tested by connecting the reactor to a glassenclosed gas system (Labsolar-6A, PerfectLight).Disperse 30 mg of the catalyst in 30 ml of pure water, and add AgNO 3 (20 mM) as an electron acceptor and 30 mg of La 2 O 3 to stabilize the pH of the reaction system.N 2 was passed through the reactor for 30 min to remove residual gas.The generated O 2 was detected under visible light irradiation (λ ≥ 420 nm) of a 300 W Xe lamp and by an online gas chromatograph (5 Å molecular sieve column, Ar carrier).

Determination of AQY efficiency
The AQY efficiency of Ni SAPs -PuCN was tested under pure water (50 mg catalyst in 50 ml solution, pH = 7, 25 °C).AQY tests were performed using a 300 W Xe lamp (PLS-FX300HU, PerfectLight) with different bandpass filters at 400, 420, 450, 500 and 550 nm (FWHM = 15 nm).The average light intensity was measured by an optical power meter (Thorlabs) and the irradiated area was controlled at 1.69 cm 2 .AQY is calculated by the following Eq.( 1): Measurement of SCC efficiency The SCC efficiency was evaluated using a 300 W Xe lamp (PLS-FX300HU, PerfectLight) with an AM 1.5 G filter as a simulated sun light source.500 mg of the catalyst was dispersed in 100 ml of pure water (pH = 7, 25 °C) with sufficient O 2 for photoreaction.The spot irradiation area is set to 1 cm 2 , and the light intensity is strictly set to 100 mW cm -2 (1 sun) by the optical power meter (Thorlabs).The SCC efficiency is calculated by the following Eq.( 2): where ΔGH 2 O 2 is the Gibbs free energy (117 KJ mol -1 ) of forming H 2 O 2 , NH 2 O 2 is the amount of H 2 O 2 produced, I is the light intensity of simulated sunlight (100 mW cm -2 ), S is the illuminated area (1 cm 2 ), and T is the illuminated time (s).

Electrochemical measurement of O 2 reduction reaction (ORR)
The number of transferred electrons (n) and H 2 O 2 selectivity of the samples in ORR were measured using a rotating ring disk electrode (RRDE).The electrochemical measurement adopts a three-electrode system, in which Pt/C is the counter electrode, RRDE is the working electrode, and Ag/AgCl is the reference electrode.The electrolyte is O 2 saturated 0.1 M KOH solution.The speed of RRDE was used at 1600 rpm and the potential range was set to 0-1.0 V vs. RHE.The catalyst ink on the RRDE working electrode is prepared as follows.After uniform grinding of 4 mg catalyst, it was ultrasonically dispersed in 400 µL pure water, 600 µL isopropanol and 10 µL Nafion solution for 1 h.Next, 10 µL of ink was dropped on the RRDE electrode and dried at room temperature.The number of transferred electrons is calculated according to the following Eq.( 3): The H 2 O 2 selectivity is calculated according to the following Eq.( 4): where I r is the ring current, I d is the disc current, and N is the collection efficiency (0.37). was collected on Shimadzu UV-3600 instrument with BaSO 4 as reference.Photoluminescence (PL) spectra were measured on a fluorescence spectrometer (Hitachi F-7000) with an excitation wavelength of 325 nm.Time-resolved photoluminescence (TRPL) spectra were collected on an Edinburgh FLS1000 fluorescence spectrometer with excitation at 375 nm.Raman spectra were acquired on a highresolution confocal Raman spectrometer (RENISHAW inVia) with an excitation laser of 785 nm.The EPR signal generated by the samples in the 5,5-dimethyl-1-pyrroline N-oxide solution (DMPO) was captured using an A300-10/12 spectrometer.The N 2 adsorption-desorption isotherm and pore size distribution of the samples were measured at 77 K using a micrometrics Max-II system.Detection of temperature programmed O 2 desorption (TPD-O 2 ) of samples using a temperatureprogrammed chemisorption instrument (AutoChem1 II 2920).The electrochemical impedance spectroscopy (EIS) was measured on the electrochemical workstation (CHI600A) with a standard threeelectrode system, in which the catalyst-coated ITO was the working electrode, the Pt wire was the counter electrode and the saturated calomel electrode was the reference electrode.

In situ XAFS measurement
The in situ XAFS measurements of Ni K-edge were carried out at the 1W1B station in the Beijing Synchrotron Radiation Facility (BSRF), China.The storage ring of BSRF was operated at 2.5 GeV with a maximum current of 250 mA.The beam from the bending magnet was monochromatized utilizing a Si (111) double-crystal monochromator and further detuning of 30% to remove higher harmonics.The photochemical in situ XAFS tests were performed by a homemade cell in a pure water solution.The XAFS spectra were collected through fluorescence mode.The Ni SAPs -PuCN catalyst on 3D substrate was cut into 1 × 2 cm 2 pieces and then sealed in a cell by Kapton film.In order to obtain information on the evolution of active sites during photochemical reactions, a series of representative working conditions were applied to the samples, including Ar-saturated solution (In Ar), O 2 saturated solution (In O 2 ) and light conditions (visible light irradiation in O 2 saturated solution).A 300 W Xe lamp (PLS-SXE 300D/DUV, PerfectLight) with a 420 nm cut-off film (λ ≥ 420 nm) was utilized as the light source for the photocatalytic reaction.During the collection of XAFS measurements, the position of the absorption edge (E 0 ) was calibrated using a standard sample of Ni, and all XAFS data were collected during one period of beam time.

XAFS data analysis
The acquired EXAFS data were processed according to the standard procedures using the ATHENA module implemented in the IFEFFIT software packages.Subsequently, k 3 -weighted χ(k) data in the k-space ranging from 2.6-11.

Computational methods
All density functional theory calculations were performed using the Perdew-Burke-Ernzerhof formulation within the generalized gradient approximation.These calculations were conducted with the Vienna Ab Initio Package 53,54 .The projected augmented wave potential was chosen to describe the ion nucleus 55 .The valence electrons were considered, using a plane wave basis set with a kinetic energy cutoff of 500 eV.Electron energies are considered self-consistent when the energy change is less than 10 -5 eV.The geometry optimization was considered converged when the force change was smaller than 0.02 eV/Å.Grimme's DFT-D3 method is used to account for dispersion interactions 56 .
The lattice constants of corrugated 2 × 2 g-C 3 N 4 monolayer in a vacuum with a depth of 20 Å was optimized, when using a 2 × 2 × 1 k-point grid for Brillouin zone sampling, to be a = 13.674Å.This model comprises of 24 C and 32 N atoms.This model and metal single-atomdoped ones were used for adsorption.The adsorption energy (E ads ) of adsorbate A was defined as: E ads = E A/surf − E surf − E A (g), where E A/surf represents the energy of the adsorbate A adsorbed on the surface, E surf and E A (g) is the energy of isolated A molecule in a cubic periodic box with a side length of 20 Å and a 1 × 1 × 1 Monkhorst-Pack k-point grid for Brillouin zone sampling, respectively.The free energy of gas-phase molecules or surface adsorbates is calculated from the equation G = E + ZPE − TS, where E is the total energy, ZPE is the zero-point energy, T is the temperature in Kelvin (298.15K is set here), and S is the entropy.The overall reaction pathway for the calculated 2e -ORR includes: (1) O 2 adsorption on the active site (*) in catalysts (Eq.5); (2) the adsorbed O 2 captures an e -and combines with H + to form *OOH (Eq.6); (3) *OOH continues the electron-coupled proton transfer reaction to form H 2 O 2 (Eq.7); (4) sites into highvalent O 1 -Ni-N 2 sites after O 2 adsorption during photoactivation.This process promotes the formation of the key intermediate *OOH, which further transforms into HOO-Ni-N 2 .Crucially, the structure of the O 1 -Ni-N 2 intermediate state ensures the end-on adsorption state of O 2 and suitable O 2 adsorption energy, leading to a fast transition from •O 2 -to •OOH.Overall, this self-optimization of Ni active site evolution (Ni-N 3 → O 1 -Ni-N 2 → HOO-Ni-N 2 ) greatly reduces the formation energy barrier of the intermediate *OOH to accelerate H 2 O 2 generation (
. The effect of Ni single atoms on O 2 selective electron transfer was investigated by electrochemical measurements on rotating ring-disk electrode (RRDE), where the disk current was derived from the reduction reaction of O 2 , while the ring current was derived from the oxidation reaction of the produced H 2 O 2 .The selectivity of H 2 O 2 generation on BCN and Ni SAPs -PuCN was monitored in O 2 -saturated 0.1 M KOH electrolyte.In Fig. 4d (bottom), the reduction disk currents of PCN and Ni SAPs -PuCN gradually increased as the potential decreased from 0.8 V (vs.RHE).In Fig. 4d (top), the ring current of Ni SAPs -PuCN was significantly higher than that of BCN, indicating that Ni SAPs -PuCN produced more H 2 O 2 .As shown in Fig. 4e, the average number of transferred electrons (n) and H 2 O 2 selectivity were calculated in the potential range of 0-0.6 V (vs.RHE).Under the same conditions, the number of transferred electrons of Ni SAPs -PuCN is closer to 2 and the selectivity of H 2 O 2 is higher than that of BCN.The number of electrons transferred on BCN was 2.77, and the H 2 O 2 selectivity was only 61.4% at 0.5 V (vs.RHE).But at the same potential, the number of electrons transferred on Ni SAPs -PuCN is 2.14, achieving 92.4% H 2 O 2 selectivity.Combined with the RRDE and photocatalytic H 2 O 2 performance tests, these fully demonstrate that Ni single-atom sites greatly improve the 2e -selectivity of O 2 and efficiently promote H 2 O 2 generation.

Fig. 4 |
Fig. 4 | Photocatalytic H 2 O 2 production activity and selectivity of Ni SAPs -PuCN.a The time course of H 2 O 2 production measured in pure water under visible light irradiation (λ ≥ 420 nm, 60 mW cm −2 ; 30 mg catalyst in 30 ml pure water, 1 g L −1 catalyst; 25 °C).Error bars are the standard deviations of three replicate measurements.b Performance comparison of all samples in pure water under visible light conditions and H 2 O 2 generation performance of Ni SAPs -PuCN under AM 1.5 G illumination (60 mW cm -2 ).c The wavelength-dependent AQY for photocatalytic H 2 O 2 production in pure water by Ni SAPs -PuCN (50 mg catalyst in 50 ml pure water, 1 g L -1 catalyst, 25 °C).d RRDE polarization curves over BCN and Ni SAPs -PuCN in O 2 -saturated 0.1 M KOH at 1600 rpm with ring current (upper part) and disk current (bottom part).e H 2 O 2 selectivity as a function of the applied potential.The inset shows the calculated average number of transferred electrons (n).f The amount of H 2 O 2 generated by Ni SAPs -PuCN under AM 1.5 G simulated sunlight irradiation (100 mW cm -2 ) and the corresponding SCC efficiency (500 mg in 100 ml pure water, 25 °C).g Summarized SCC efficiencies of recently reported photocatalysts (g-C 3 N 4based and other types of photocatalysts) for H 2 O 2 production in pure water.

2 Fig. 5 |
Fig. 5 | In situ structural evolution of Ni sites in photocatalytic 2e -ORR.a Ni Kedge XANES spectra of Ni SAPs -PuCN during photocatalytic 2e -ORR in Ar or O 2 saturated aqueous solution at room temperature.The inset is the enlarged Ni Kedge XANES spectra.b FT-EXAFS spectra of Ni SAPs -PuCN under in situ operation.c Schematic diagram of Ni sites structure evolution of Ni SAPs -PuCN in the photocatalytic 2e -ORR.d Optimized structures and charge difference density of adsorbed O 2 molecule on Ni SAPs -PuCN (The isosurface value is 0.0016 eV Å −3 , electron accumulation and consumption are indicated in yellow and blue, respectively).e First-shell fitting of FT-EXAFS spectra for Ni SAPs -PuCN in O 2 saturated aqueous solution.f, g EPR signals of •O 2 -and •OOH of the samples in the presence of DMPO.h Raman spectra of Ni SAPs -PuCN recorded during the photoreaction in O 2saturated aqueous solution.

Fig. 6 |
Fig. 6 | Catalytic enhancement mechanism of Ni sites evolution in Ni SAPs -PuCN.a Schematic diagram of O 2 adsorption structure on metal sites (upper part), and optimized structures of adsorbed O 2 molecule on BCN and Ni SAPs -PuCN (bottom part).b Comparison of O 2 adsorption energy and charge difference density on BCN and Ni SAPs -PuCN.c Temperature programmed O 2 desorption (TPD-O 2 ) profiles of BCN and Ni SAPs -PuCN.d O 2 dissociation energies and profile corresponding to Ni SAPs -PuCN side-on (upper part) and end-on O 2 (bottom part) adsorption configurations.e The PDOS plots for free O 2 , Ni SAPs -PuCN and Ni SAPs -PuCN after end-on O 2 adsorption.f Free energy profiles for photocatalytic H 2 O 2 evolution reactions over BCN and Ni SAPs -PuCN.Optimized models and charge difference density after BCN (g) and Ni SAPs -PuCN (h) generate *OOH (The isosurface value is 0.0016 eV Å −3 .Electron accumulation and consumption are indicated in yellow and blue, respectively).
), indicating that Ni single atoms can efficiently adsorb O 2 .The charge difference density after O 2 adsorption on the two samples (inset of Fig. 6b) was further calculated and analyzed.Compared to BCN (the bond length of adsorbed O 2 on BCN is 1.23 Å), the adsorbed O 2 on Ni SAPs -PuCN has a longer O=O bond length (1.29 Å) and a larger charge transfer, indicating that the O 1 -Ni-N 2 intermediate structure promotes the O 2 adsorption and activation.Furthermore, temperature-programmed desorption of O 2 (TPD-O 2 , Fig. 6c) suggested that Ni SAPs -PuCN has remarkable stronger O 2 adsorption capacity than BCN, which fully confirms our calculation results.In addition, maintaining the O=O bond of O 2 and avoiding the dissociation of O 2 are important prerequisites for H 2 O 2 generation in 2e -ORR 4,52 .Different O 2 adsorption configurations affect the extent of O 2 activation and the subsequent reactions, and these effects were investigated through theoretical calculations.As shown in Fig. 6d, the O 2 dissociation barriers for side-on (O 2 -Ni-N 2 ) and end-on O 2 adsorption configurations (O 1 -Ni-N 2 ) on Ni SAPs -PuCN are 2.00 eV and 2.86 eV, respectively.This indicates that the side-on adsorbed O 2 -Ni-N 2 is more inclined to dissociate O 2 , while the end-on adsorbed O 1 -Ni-N 2 intermediate structure inhibits the dissociation of O 2 and favors the H 2 O 2 generation, further emphasizing the importance of O 1 -Ni-N 2 in the structural evolution.The PDOS was further used to elucidate the charge transfer between the end-on adsorbed O 2 molecule and Ni SAPs -PuCN.As shown in Fig. 6e, when free O 2 with a spin triplet ground state is adsorbed onto Ni SAPs -PuCN, the 2p orbitals of O exhibit noticeable hybridization with the Ni 3d orbitals of Ni SAPs -PuCN.The O 2 molecule donates the p-electron to the empty 3d orbital of the Ni atom, and then the 3d electron from the Ni atom is donated back to the 2π * orbital of O 2 .As a result, the original empty 2π * orbitals in the spin-down channel of free O 2 are partially occupied, leading to the elongation of the O = O bond length.Based on the above analysis, the improved O 2 adsorption and activation properties of Ni active sites provide strong support for the formation of O 1 -Ni-N 2 sites in in situ XAFS.Moreover, the critical intermediate state with the O 1 -Ni-N 2 structure ensures suitable O 2 adsorption energy and adsorption configuration, inhibits O 2 dissociation, and improves the selectivity, thus guaranteeing the subsequent efficient H 2 O 2 generation.
. The enhanced O 2 adsorption capacity of Ni SAPs -PuCN further facilitates *OOH formation, thus evidently promoting the conversion of *OOH to H 2 O 2 .Moreover, Fig. 6g-h shows the charge density difference between the two structures and the resulting *OOH.Contrast to BCN, the charge redistribution between *OOH and Ni SAPs -PuCN is more remarkable, reflecting that Ni sites effectively promote the generation of key intermediate *OOH, which is also in line with the EPR test (Fig. 5g).Combined with the •O 2 -and •OOH radical tests of EPR (Fig. 5f-g), the highly active Ni SAPs -PuCN has the strongest •O 2 -and •OOH signals in contrast, indicating that the evolution of the Ni site structure is accompanied by a rapid transformation mechanism from •O 2 -to •OOH, which strongly supports the above calculations.More importantly, theoretical simulations also confirm that the structural evolution of Ni sites (structure diagram in Fig. 6f) during this process is consistent with in situ experiments: from Ni-N 3 to O 1 -Ni-N 2 to HOO-Ni-N 2 , corresponding to the reaction mechanism from O 2 to •O 2 -to •OOH to H 2 O 2 .This dynamic structural evolution mechanism of Ni single atoms represents the self-optimization of active sites in 2e -ORR surface reaction, which is the core factor for Ni SAPs -PuCN with high activity and high selectivity for H 2 O 2 .

8 Å − 1
were Fourier transformed to real space using a Hanning window (dk = 1.0 Å −1 ) to separate the EXAFS contributions from different coordination shells.The best background removal was at the R bkg = 1.0 Å, and the low-frequency noise was removed fully.As for the Ni SAPs -PuCN under saturated Ar solution, the curve fitting was done on the k 3 -weighted EXAFS function χ(k) data in the k-range of 2.6-11.8Å −1 and in the R-range of 1.0-2.2Å.The number of independent points for these samples are N ipt = 2Δk·ΔR/ π = 2 × (11.8 − 2.6) × (2.2 − 1.0)/π = 7.However, the first coordination peak of Ni SAPs -PuCN under O 2 saturated solution conditions showed similar intensity and higher R shift compared with Ar-saturated solution, which was ascribed to the fracture of Ni-N bond and addition of Ni-O coordination.Therefore, the two subshells of Ni-N and Ni-O coordination were considered for the curve fitting of Ni SAPs -PuCN under O 2 saturated solution.During curve fittings, each of the Debye-Waller factors (σ 2 ), coordination numbers (N), interatomic distances (R), and energy shift (ΔE 0 ) was treated as adjustable parameters.